The present invention relates to machines for the rotational molding of plastics and in particular to spindle arms used therein. Attention is diverted to my copending application, Ser. No. 822,800 filed Jan. 27, 1986 of which this invention forms a part and the disclosure of which is incorporated herein by reference.
Rotational molding machines are particularly well adapted for the manufacture of large parts from vinyl plastisols. The specific materials which work well in conjunction with rotational molding machines include thermoplastic polyethylene powders and in particular low density polyethylene. Other forms of thermoplastic powders which have been commercially used with rotational molding machines include low and high density polyethylene, polyethylene copolymers, cellulose acetate-butyrate, vinyl dry blends, impact styrenes and high-modulate thermoplastics. These materials are introduced into a mold as either a heat convertible liquid or as a powder, wherein the rotational molding process changes the physical form of the material to a continuous solid. This change occurs while the mold is heated and simultaneously rotated about two perpendicular axes within the machine.
A rotational molding machine may be adapted to hold one or a plurality of molds during the rotational molding process, the molds being held within the machine on a framework called a spider. The spider is attached to a spindle arm, which spindle arm imparts the necessary two-axis drive to the spider during the molding process. After the heated rotation the molds are cooled, unloaded from the spider and the finished parts are then removed or "stripped" from the molds. The molds can then be recharged with fresh plastic and loaded onto the spider to repeat the process.
Rotational molding machines are generally classified as batch-type machines or continuous machines. Continuous rotational molding machines typically are semi-automatic or fully automated, having a plurality of driven spindles upon which the spiders are attached with the path of travel of the spiders passing through various processing stations which permit loading, heating, cooling and stripping of the molds so that the machine operates more or less continuously. The batch-type rotational molding machine, to which type of machine the present invention is directed, is generally less expensive than a continuous machine and typically utilizes a rotatable spindle permanently mounted within an oven, wherein the loading, heating, cooling and stripping process steps are performed. With some machines the spiders can be removed from the spindle and moved manually from the oven to a cooling, unloading and stripping station, so that some of these operational process steps may be performed outside of the oven. However, as temperatures of up to 1200.degree. F. can be required during the rotational heating process, considerable time can be required for the spiders to cool sufficiently to permit their detachment from the spindle. This cooling time represents time during which the oven and spindle cannot be used to process other parts and therefore batch-type machines generally have a lower production rate than do continuous machines.
In batch-type rotational molding machines it is desirable to handle and process molds of widely varying dimensional configurations, in order to facilitate the manufacture of plastic parts of different sizes and shapes. In accomplishing this purpose it is frequently desirable to remove the entire rotational arm assembly from the machine and substitute therefore an offset rotational arm, capable of handling different and larger mold configurations. Accordingly, there is a need for a rotational arm and spindle that can be readily removed from a batch-type rotational molding machine so that a different configuration rotational arm and spindle assembly can be quickly installed. It is desirable that this removal operation be performed even while the molds are still attached to the rotational arm assembly so as to reduce equipment downtime between batch manufacturing steps.